Hi, with the help of Guille I noticed that normal instances usually register weakly: TestRunner >> initialize ..... SystemAnnouncer current weak on: ClassAdded, CategoryAdded, ClassRemoved, CategoryRemoved, ClassRenamed, CategoryRenamed, ClassRecategorized send: #update to: self. ..... and classes don't: TestCase class >> registerForNotifications ... SystemAnnouncer current on: MethodAdded, MethodModified, MethodRemoved send: #methodChanged: to: self. so..Guille told me this is because usually the cycle-life of instances is short in comparisson with classes. But classes can be removed as well. What happen then? the announcer will hold always a reference to an obsolete class? my question is, should we always use weakly? if not, why ? thanks for the explanation, -- Mariano http://marianopeck.wordpress.com